In contrast to the conventional process for making gypsum fiberboard in which calcium sulfate dihydrate is used as a starting material, the process described above has some important advantages. In making the mash and in molding the board, the gypsum is present as the dihydrate and is thus not settable. There is thus no concern; without transformation over any setting time and no danger of the production equipment being encrusted or clogged by setting gypsum. Moreover it is possible to feed back waste products or scrap from the manufacturing process. The economic value of the process is limited because the setting process is extremely slow.
This process is described in German Open Patent Application No. 28 16 466. In this process the rehydration occurs as follows: The board resides in a chamber or space in which the humidity or atmospheric moisture content is from 80 to about 100% for a duration of 2 hours to seven days. For accelerated rehydration a setting accelerating agent, e.g. KAl(SO.sub.4).sub.2 can be mixed or added to the rough molded body prior to heating. Also it is possible after heating to apply a setting accelerating agent by soaking.
Extensive experiments have shown that as a practical matter the setting time has an order of magnitude of about one week. However by adding an accelerating agent on average the setting time is somewhat reduced, however the results are disappointingly poor. It has proven up to now to be impossible to attain in a reproducible way a practical lower limit for the setting time or even to approach it.
In German Patent No. 34 19 558, which builds on these processes, the process is provided with a combination of features whereby the setting time is shortened. The mash is adjusted to a pH value which is less than 7 and the molded boards are treated in a pressurized chamber between 3 and 50 minutes in a saturated steam atmosphere at 130.degree. to 170.degree. C. and after that are cooled in the pressurized chamber by releasing steam.
In practice with the process of German Patent No. 34 19 558 one can scarcely guarantee a setting time of less than 50 hours. The slow setting conditions are difficult to explain in view of the fast setting rates which one expects with the hemihydrate. Although a delaying action of the fiber additive has long been known, the extent of this undesirable effect is surprising. Perhaps setting retardant substances unknown up to now are released from the fiber as a result of heating. In comparable processes which operate without fiber additives no similar effect is evident. This can be seen from the following examples.
German Open Patent Application No. 31 17 662 describes a process for making gypsum building blocks in which a calcium sulfate dihydrate raw material is pressed in a moist state, treated hydrothermally at a temperature up to 473K and converted and subsequently cooled. The raw material can contain also diatomaceous earth (Kieselguhr), fly ash and additional additive materials, such as sand. Thus rehydration and recrystallization occur during cooling, apparently in the open, so that the building blocks already have a considerable hardness after cooling. In this respect the building blocks which contain no fiber, behave completely differently from fiberboard.
The German Open Patent Application No. 28 18 169 relates to a process for making gypsum articles, especially facing board or wall boards. These are formed by addition of water to natural gypsum dihydrate, subjecting the boards to a thermal treatment at 100.degree. to 200.degree. C., cooling and moisturizing. The moistening can occur by immersing in water, spraying with water or saturating with steam or water vapor. Subsequently the parts are dried over the course of about three hours. Also these results cannot be transferred to fiber containing boards according to our experience.
The German Open Patent Application No. 26 49 300 teaches a process for making fiberboard like materials based on calcium sulfate dihydrate in which the calcium sulfate dihydrate is mixed with a fiberous material in the presence of water, subsequently molded while being dewatered and then dried at 95.degree. to 140.degree. C. A recrystallization of gypsum is not required.
In this process first a slurry of water, calcium sulfate dihydrate and fiberous material is made and this slurry is mixed with a binding agent, e.g. a completely synthetic binding agent, combined with a modified natural material, e.g. modified starch and carboxymethylcellulose. As a result the union is improved between the gypsum; and the fiber containing material.
Experience has shown that the strengthening action of this additive remains below expectations according to the time and temperature of the heating process. Unsatisfactory results then occur above all when, e.g. according to German Open Patent Application No. 28 16 466, the temperature is raised above the range given in German Open Patent Application No. 26 49 300 to a temperature 180.degree. C. and the heating is performed in an autoclave at an increased pressure until the entire calcium sulfate dihydrate is converted to calcium sulfate hemihydrate.
The reduced action of the binding additive is possibly attributed to the fact that the binding agent is changed by the heat treatment. It is, for example, known that starch under the influence of pressure and temperature is converted into a starch sugar in acid medium in a period of hours.